For numerous applications, it is preferable to use a transmission mode authorizing simultaneous transmission in both stations, for better utilization of the lines.
An example of a particularly suitable application is the interconnection among the units of a central information process subsystem including a large number of units. When the number of processors and memories becomes large, the selection of a bus to effect the exchanges of information between all these units is no longer appropriate. It is preferable in this context to use serial links, connecting each unit to all the other units of the system in such a way as to increase the parallelism. Considering the large number of links required in such a system, the choice of bi-directional links is preferable.
Such bi-directional transmission necessitates the presence in each station of a transceiver connected to the lines by adaptation impedances. The receiving part of the transceiver includes means that can handle collisions, that is, simultaneous transmission over the common link by the transmitters of two stations connected to one another. These means must under all circumstances permit the detection, from the electrical state of the line, of the existence and nature of the signals transmitted by the remote station.
To solve this problem, the principle typically used is that of compensation in the receiver for the effect produced on the line by the transmitter of the same station when it is transmitting. Thus in the case of transmission, a subtraction of the signal present on the line from the transmission signal is performed. The difference obtained is then representative of the signal transmitted by the transmitter of the remote station. This embodiment is for instance described in European Patent Application EP-A-186142, published Jul. 2, 1986, entitled "Two wire bidirectional digital transmission system".
Another solution consists not in subtracting the transmission signal from the line signal when the transmitter of the station in question is transmitting, but contrarily to add the line signal, when the station is not transmitting, to a compensation signal of the same polarity, suitably chosen so that the resultant sum will be representative of the only transmission signal of the other station.
It is understood that these transmission and reception means are embodied by means of electronic circuits that have to be powered electrically. To obtain fast circuits, technologies that use bipolar transistors, such as ECL (emitter-coupled logic), are chosen. As a result, the consumption of energy by the circuits at rest is not insignificant and may cause exaggerated heating of the circuits. In the example of interconnection among a large number of units, integrated circuits are thus advantageously used to make the interface between each unit and the other units of the systems. Each integrated circuit can then include a large number of transceivers, among which only some of them are active simultaneously. All those that are at rest will then increase the useless power consumption and heating.
To solve this problem, it is accordingly desirable to be able to cut the power to the transceivers, or at least their power circuits, when they are not used for either transmission or reception.
However, the capability of cutting the power to the transceivers must nevertheless still enable reliable transmission operation. Thus in a station in question, where a power of the transceiver is cut, it must always be possible to detect whether the station to which this transceiver is being connected is transmitting a signal or not, because if a signal is transmitted, then the supply to the transceiver in question must immediately be re-established.